Technical Field
The present invention relates to a spring forming device in which a spring such as a coil spring is continuously hot-formed while feeding a steel wire, and in particular, it relates to a technique in which uneven heating of the steel wire is decreased by continuously cutting the steel wire without stopping the feeding thereof.
Background Art
Recently, in transportation equipment, in particular, in an automobile, requirements for reducing fuel consumption are annually increased in consideration of global warming, and size reduction and weight reduction of automobile parts is required more stringently than ever. In order to satisfy this requirement for size reduction and weight reduction, for example, in compression coil spring parts such as a valve spring used in an engine, a clutch damper spring used in a clutch, etc., attempts were made up until now to improve fatigue resistance or settling resistance, which are important characteristics of the coiled spring, by strengthening or surface-treating materials.
Generally comparatively small coiled springs such as a valve spring, a clutch damper spring, etc., are produced by cold-forming using coil material. In contrast, comparatively large springs such as a suspension spring are generally produced by hot-forming using bar material. This is one reason it is difficult to form since workability in the cold-forming is low due to thick wires being used.
None of them can be selected as better forming, since there are some advantages and some disadvantages in the cold-forming and the hot-forming of the coiled spring. For example, in a coiled spring in a shape which can be cold-formed since a wire diameter is comparatively small or a spring index is large, the cold-forming is generally adopted from the viewpoint of the ease of processing technique and the mass-productiveness (tact, dimensional accuracy, cost) due to machining speed, equipment cost, etc. Additionally, in the cold-forming, a forming technique using coreless material is established, and high flexibility of a shape of the coiled spring is also one of the large causes of use of the cold-forming. In general, springs of a valve spring class are produced by cold-forming.
On the other hand, hot-forming has an advantage in which coiling distortion does not occur in processing, in comparison with the cold-forming, and when a wire diameter d is large or when a spring index D/d which is a ratio of a coil average diameter D per a wire diameter d is small, it is used in forming of the coiled spring which cannot be cold-formed due to low workability. However, in the hot-forming, it is necessary to form a coiled spring in a coiled shape by winding around a core bar since the material is soft. Therefore, the flexibility of the shape is low, and moreover, the core bar must be arranged in each product.
In hot-forming, a shape or performance of the product is greatly affected by heating temperature in forming of the steel wire. Therefore, in order to maintain the quality (form accuracy, grain size) of the product, it is desirable that the steel wire be formed in a state in which it is uniformly heated over the whole. That is, it is desirable that the feeding speed of the steel wire, which affects the heating temperature, be made more uniform.
Patent Document 1 discloses a mechanism in which a motor for driving a cutting tool carries out a reciprocating motion and a rotary motion, a coiled spring is cut in not only a forward motion but also a backward motion of the motor for driving a cutting tool, and therefore, the coiled spring can be cut at a higher speed.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 2008-080386.